Sheep study poses sexuality questions

They say a region of the brain involved in sexual behaviour is different in "gay" rams which prefer to mate with other males.

The findings are similar to those identified by the neuroscientist Simon LeVay in the brains of gay men.

Controversy has surrounded the research, partly because many of the men had died of AIDS.

It was not clear whether the differences were related to the disease or to sexual preferences.

The part of the brain analysed in both studies is an area of the hypothalmus involved in mating behaviour, the preoptic hypothalamus.

In humans and some other animals it is about twice as large in males compared with females and contains twice the number of cells. Its function in behaviour is not fully known.

Researchers looked at sheep in an attempt to understand the biological basis of sexual behaviours.

They say previous studies have shown that between six and 10% of rams are attracted to males rather than females.

They analysed the brain structures of 17 rams, nine of which preferred to mate with males, and 10 ewes.

Human sexuality

Research focused on a group of brain cells in the preoptic hypothalamus called the sexually dimorphic nucleus.

"Interestingly, this bundle of neurons is smaller in ewes and in rams with same-sex preferences than it is in rams that prefer ewes," said lead researcher Dr Kay Larkin.

"We also determined that the volume of the sexually dimorphic area is approximately the same in rams that prefer rams as it is in ewes."

The researchers believe sheep could help provide clues about human sexuality.

Professor Charles Roselli said: "While we realise that sexuality is more complex in humans than reproductive behaviours in sheep, this model will help illuminate the basic principles that apply to all mammals, and may be helpful in understanding the biology of human behaviours as well."

He said the studies show there is a biological mechanism involved in partner preference. But he said the motivation for the work was purely scientific and he did not set out to prove whether sexual orientation was influenced by nature or nurture.

"I'm not trying to fuel that debate but I'm sure that people will use it as part of the nature part of that debate," he told BBC News Online.

There has been conflicting evidence on whether biology might underpin homosexuality to some extent.

David Allison of the UK gay rights group Outrage says there is nothing wrong with the so-called nature/nurture debate; what matters is getting rid of prejudice.

"It's the prejudice that is wrong not how gay people come into world," he told BBC News Online.